Considerable evidence suggests that cell surface lectins on cellular slime molds are involved in intercellular adhesion. Binding studies have established the existence of high affinity receptors for the endogenous lectins on the surface of differentiated amoebae. In Polysphondylium pallidum, we have found a potent inhibitor of the endogenous lectin pallidin in an extract of membranes. We have purified this inhibitor and have identified it as a high molecular weight heterosaccharide, consisting predominantly of rhamnose and glucose. This receptor candidate blocks agglutination of P. pallidum cells at high concentrations and augments agglutination at low concentrations. We will prepare antibodies against this inhibitor and determine 1) its localization within amoebae; and 2) whether adhesion is blocked by univalent antibodies. In teratocarcinoma cells, we have solubilized a cell surface lectin activity with specificity for sulfated fucose polymers. We will purify this lectin, raise antibodies against it, and study its role in intercellular adhesion. In cultured BHK fibroblasts we have identified a cell surface lectin that mediates binding to certain erythrocytes. We will define the specificity of this lectin and investigate its potential functions.